1. Field of the Invention
The present invention relates to a linear compressor and a method for controlling the same, and more particularly to a linear compressor which can automatically short-circuit part of a coil having no power-supply voltage in order to substantially prevent an excessive stroke from being generated when the linear compressor is initially operated, and a method for controlling the same.
2. Description of the Related Art
Typically, compressors are machines used to compress fluid, such as air, refrigerant gas, etc. Among them, in case of a linear compressor, driving power of a linear motor is transmitted to a piston of the compressor so that the piston rectilinearly reciprocates inside a cylinder, thereby sucking and compressing the refrigerant gas. The linear compressor generally comprises a compressing unit for compressing the refrigerant gas, and a driving unit for providing the compressing unit with the driving power to drive the compressing unit.
FIG. 1 is a cross sectional view illustrating a conventional linear compressor.
As shown in FIG. 1, the conventional linear compressor comprises: a power-supply unit (not shown) for providing a power-supply voltage; a hermetic casing 1, to one side of which is connected a pipe (not shown) for use in the suction of a refrigerant; a cylinder 2 fixedly disposed inside the hermetic casing 1, and internally defining a compression space for use in the compression of the refrigerant; a piston 3 installed to rectilinearly reciprocate inside the cylinder 2 for sucking and compressing the refrigerant in the compression space; and a linear motor 6 connected to a front end of the piston 3 for providing the piston 3 with driving power to allow the piston 3 to rectilinearly reciprocate.
The linear compressor further comprises: a suction valve 4 installed at a rear end of the piston 3 for sucking the refrigerant into the compression space defined between the cylinder 2 and the piston 3; and a discharge valve assembly installed at a rear end of the cylinder 2 for discharging the refrigerant from the compression space to the outside.
In this case, the linear motor 6 consists of a stator, and a mover. The stator comprises a cylindrical outer core 6a, a cylindrical inner core 6b loosely inserted in the outer core 6a to form a predetermined gap therebetween, and a coil assembly 6c positioned between the outer core 6a and the inner core 6b. 
The mover comprises a magnet 6d positioned between the inner core 6b and the coil assembly 6c in a rectilinearly reciprocable manner, and a magnet frame 6e used to connect and fix the magnet 6d and the piston 3 to each other for allowing rectilinear reciprocating motion of the magnet 6d to be transmitted to the piston 3.
With the conventional linear compressor configured as stated above, upon receiving a power-supply voltage from the power-supply unit, a current flows in a coil of the coil assembly 6c, and creates a magnetic field around the coil assembly 6c. As the magnetic field interacts with the magnet 6d, inducing rectilinear reciprocating motion of the magnet 6d. 
In this case, the magnet frame 6e also rectilinearly reciprocates along with the magnet 6d, allowing the piston 3 to rectilinearly reciprocate inside the cylinder 2.
At the same time the piston 3 rectilinearly reciprocates inside the cylinder 2, the refrigerant gas enters into the hermetic casing 1 according to operations of the suction valve 4 and the discharge valve 5. The refrigerant gas is first sucked into the cylinder 2 through an inner through-bore of the inner core 6b and a refrigerant passage of the piston 3, and compressed in the compression space inside the cylinder 2. Then, the compressed high-pressure and high-temperature refrigerant gas is discharged from the cylinder 2, and finally discharged to the outside of the hermetic casing 1 through a discharge pipe (not shown).
However, in the above-identified linear compressor, if the power-supply voltage is applied to only part of the coil when the power-supply unit is initially connected, an induced current is generated in the remaining part of the coil having no power-supply voltage, unavoidably creating a force hindering the motion of the piston 3. Such a force excessively increases the stroke of the piston 3, causing the piston 3 to collide with the discharge valve 5.
The induced current will hereinafter be described in detail.
Typically, if a coil is fixed in place and a magnet located around the coil is moved, or if the magnet is fixed in place and the coil is moved, a current is generated in the coil. Further, when the magnet is moved close to the coil or far from the coil, or when polarity of the magnet is changed, the flow direction of the current generated in the coil is changed.
Such a current induction phenomenon caused by relative motions between the coil and the magnet is called electromagnetic induction, and electromotive force generated in both ends of the coil is called induced electromotive force. In this case, the current flowing in the coil under the influence of the induced electromotive force is called an induced current.
If an ammeter is connected to a closed circuit connecting the power-supply unit to the coil, the ammeter's scale does not immediately indicate a specific numerical value, and gradually moves until reaching a predetermined numerical value due to the induced current. If the intensity of the induced current is changed, the strength of a magnetic field produced around the coil is also changed.
In more detail, if the intensity of the current flowing in the coil is changed, magnetic flux flowing in the coil is also changed, such that the induced electromotive force is generated. In this way, due to the change of the current flowing in the coil, the induced electromotive force is generated in the coil, and at the same time, the induced current flows in the coil. This phenomenon is called self-induction, and the generated induced electromotive force is represented by the following Equation 1:
                    V        =                              -            L                    ⁢                                    Δ              ⁢                                                          ⁢              I                                      Δ              ⁢                                                          ⁢              t                                                          [                  Equation          ⁢                                          ⁢          1                ]            where “L” serving as a proportional constant is a self-induction coefficient. The self-induction coefficient is proportional to a variety of factors, for example, magnetic permeability of an iron core inside the coil, the number of turns of the coil, and a cross section of the coil, and is inversely proportional to the length of the coil.
Therefore, as can be seen from the above Equation 1, in part of the coil having no power-supply voltage is generated the induced current flowing in an opposite direction of the current, which flows in the remaining part of the coil connected to the power-supply unit, and this generates a larger stroke than the existing stroke, resulting in a considerable increase in noise during operation of the linear compressor.
In order to solve the above problem, it has been proposed to install a drive capable of adjusting the amount of current applied to the motor. Although such a solution can reduce the amount of current required when the linear compressor is initially operated, it requires additional control parts, increasing manufacturing costs of the linear compressor.